The present invention relates to a thermosensitive recording material; more specifically, it relates to a diazo-type thermosensitive recording material in which the images can be fixed, i.e., protected against tampering and accidental obliteration, and in which the recording material itself can be stabilized so as to prevent additional imaging thereon.
Conventionally, there are known thermosensitive materials which utilize the reaction between an organic metal salt and a reducing agent for thermosensitive coloring, or which utilize the reaction of leuco dye with an acidic material for such coloring. However, these thermosensitive recording materials are not suitable for image fixing and the recorded images cannot be made permanent. That is, with these non-fixable type thermosensitive recording materials, it is possible to record additions to the already recorded information, and, thus, recorded matter on such a thermosensitive recording material can be altered, which may lead to low reliability of the record. Therefore, such a thermosensitive recording material cannot be used for recording many kinds of items or making documents in fields where reliability is of high importance, such as for securities, merchandise coupons, entrance tickets, certificates or payment slips, etc.
Furthermore, in general, if this kind of recording material is exposed to a temperature above 70.degree. C., the entire surface becomes darkly colored and the record cannot be read. And in the case of a thermosensitive recording material which utilizes the reaction of leuco dye with an acidic material, the recorded matter thereon disappears when exposed to organic solvents or plasticizers contained in plastic materials.
The above-mentioned shortcomings all stem from the fact that those thermosensitive recording materials are of a non-fixable type.
On the other hand, there is known a recording material which utilizes the reaction between a diazo compound and a coupler, the so-called diazo photosensitive paper. With this diazo-type photosensitive paper, it is possible to fix the record by decomposing the residual diazo compound with light irradiation thereon.
There is known another thermosensitive recording material which also utilizes the reaction of a diazo compound with a coupler. This thermosensitive recording material of the diazo-type uses a diazo compound, a coupler and a basic material in the reaction mixture. When the recording material is preserved, the coloring reaction is prevented by separating any one of the components from the others.
The following methods have been proposed for such a separation of a component.
(1) Separation of microcapsule.
(2) Separation by putting at least one of the three components (the diazo compound, the coupler or the basic substance) into a thermosensitive material having a melting point or softening point between 50.degree. C. and 250.degree. C.
However, a diazo-type thermosensitive recording material formed with the above-mentioned separation methods does not provide satisfactory quality for practical use. In the method (1), if capsulation is incomplete, the recording materials that can be obtained have poor preservability and poor coloring performance. In the case of the method (2), the reacting components may become exposed on the surface of the recording material during the production process, in which those components are granulated after being mixed with other components of the thermosensitive recording material, so that the preservation thereof is poor, and coloring cannot be effected by flash heating.
Recently, a thermosensitive recording material has been sought which would allow rapid recording in facsimile and telex apparatus, computers and medical analytical instruments. Especially in the field of facsimiles, a thermosensitive recording material suitable for rapid recording is required so that communication costs can be minimized.
Although a diazo-type thermosensitive recording material has the advantage of being fixable, the hitherto known materials are not satisfactory for long-term preservation and have insufficient thermal response to a thermal head.